Behind the scenes

As revealed by J. K. Rowling on Pottermore, this character was originally named "Katrina", but this was crossed out on her list of the forty students in Harry's year and replaced with "Isobel". Additionally, it has been revealed that Minerva McGonagall's mother was named Isobel, and Minerva had fallen in love with a Muggle named Dougal.[1]

As an Isobel McDougal was never featured in a published book in the series, but a Morag MacDougal was, it is possible that the two are siblings (due to the name), though it is debated if the two are independent characters or Isobel was simply renamed Morag.

Etymology

"Dougall" is from the Gaelic Dubhgall, or "dark stranger," and "Mac" is "son of."